Abstract During Phase I, we will test the feasibility of developing a magnetographic camera technology as a new tool in neuroscience to facilitate the detailed analysis of electrical currents in diverse neuronal circuits. Instead of photographic images, the camera will produce single-shot images of the magnetic fields from in vivo samples of interest, such as a patch of the grey matter in the cerebral cortex or the dorsal hippocampus. The camera will be based on the room-temperature microfabricated optically-pumped magnetometer (OPM) technology that we have been developing during the past decade. Our stand-alone single OPMs have noise levels comparable to the magnetometers based on superconducting technology, but we have not yet used OPMs to develop a multi- pixel camera. We believe they can be used to build a camera with more than one megapixels, providing high spatial and temporal resolution. In Aim 1, we will build a bench-top proof-of-concept camera system operating in a magnetically shielded room. The OPM transduces each magnetic image into an optical image that is detected by the CMOS detector at 1,000 fps or greater. Our preliminary results have noise levels of <400 fT/?Hz for a 50 ?m2 pixel size and <80 fT/?Hz for a 250 ?m2 pixel. In Phase I, we will improve the noise level of the detector by a factor of ~2 and will improve our detector design to reduce the current 4 mm gap between the sample and detector. Our target Phase II goal is to reduce this gap to 250 m. In Aim 2, we will evaluate the possibility of using the camera in combination with an array of electrodes to analyze neuronal currents in a novel way. In neurophysiological studies, the emphasis has been placed almost exclusively on analyzing the neuronal interactions along the radial direction in the cortex, perpendicular to the cortical surface. The physiology of the horizontal connections parallel to the cortical surface has been much less emphasized. A high-density 3D array of intracortical electrodes can in principle be used to determine both types of currents from the potential measurements. However, this is quite complex with large uncertainty. The horizontal and radial circuitries can be decomposed and separately analyzed by using the camera with an electrode array since the camera will preferentially see the B fields produced by the horizontal circuit, while the electrode array will see both. Together, these two types of circuitry can be analyzed to provide a better understanding of the cortical circuit. We will determine how well the Phase I camera can determine the horizontal circuitry in a simulation study and in an in vitro intact isolated lissencephalic cerebellum of a turtle with the well-known parallel fiber system in the superficial molecular layer running parallel to the cortical surface. In Phase II, we plan to develop a completely self-contained portable multi-pixel camera with a magnetic shield inside the camera casing so that it can be used anywhere without an extra shielding. Its usefulness will be evaluated for studying cortical and hippocampal physiology in in vivo rats. Since the camera uses the MEMS technology, it can be eventually mass produced economically for wide use in neuroscience.